This invention relates to electric cables, and methods of manufacturing and using such cables. In one aspect, the invention relates to electric cables with improved armor wires used with wellbore devices to analyze geologic formations adjacent a wellbore, methods of manufacturing same, as well as uses of such cables.
Generally, geologic formations within the earth that contain oil and/or petroleum gas have properties that may be linked with the ability of the formations to contain such products. For example, formations that contain oil or petroleum gas have higher electrical resistivity than those that contain water. Formations generally comprising sandstone or limestone may contain oil or petroleum gas. Formations generally comprising shale, which may also encapsulate oil-bearing formations, may have porosities much greater than that of sandstone or limestone, but, because the grain size of shale is very small, it may be very difficult to remove the oil or gas trapped therein. Accordingly, it may be desirable to measure various characteristics of the geologic formations adjacent to a well before completion to help in determining the location of an oil- and/or petroleum gas-bearing formation as well as the amount of oil and/or petroleum gas trapped within the formation.
Logging tools, which are generally long, pipe-shaped devices may be lowered into the well to measure such characteristics at different depths along the well. These logging tools may include gamma-ray emitters/receivers, caliper devices, resistivity-measuring devices, neutron emitters/receivers, and the like, which are used to sense characteristics of the formations adjacent the well. A wireline cable connects the logging tool with one or more electrical power sources and data analysis equipment at the earth's surface, as well as providing structural support to the logging tools as they are lowered and raised through the well. Generally, the wireline cable is spooled out of a truck, over a pulley, and down into the well.
Wireline cables are typically formed from a combination of metallic conductors, insulative material, filler materials, jackets, and metallic armor wires. Armor wires typically perform many functions in wireline cables, including protecting the electrical core from the mechanical abuse seen in typical downhole environment, and providing mechanical strength to the cable to carry the load of the tool string and the cable itself.
Armor wire performance is heavily dependent on corrosion protection. Harmful fluids in the downhole environment may cause armor wire corrosion, and once the armor wire begins to rust, strength and pliability may be quickly compromised. Although the cable core may still remain functional, it is not economically feasible to replace the armor wire(s), and the entire cable typically must be discarded.
Conventionally, wellbore electrical cables utilize galvanized steel armor wires (typically plain carbon steels in the range AISI 1065 and 1085), known in the art as Galvanized Improved Plow Steel (GIPS) armor wires, which do provide high strength. Such armor wires are typically constructed of cold-drawn pearlitic steel coated with zinc for moderate corrosion protection. The GIPS armor wires are protected by a zinc hot-dip coating that acts as a sacrificial layer when the wires are exposed to moderate environments.
While zinc protects the steel at moderate conditions and temperatures, it is known that corrosion is readily possible at elevated temperatures and certain aggressive “sour well” downhole conditions. Hence, in such environments the typical useful life of a cable is limited, and the cable may be easily compromised. Also, hot dip galvanization results in a decreased steel strength and increases potential fracture origin sites, which may further contribute to corrosion related GIPS armor wire failure.
Further, during hot-dip galvanization an intermediate zinc-iron alloy layer forms between the steel and zinc. Because steel, zinc-iron alloys, and zinc all have different thermal expansion coefficients, this may lead to formation of cracks in the zinc-iron alloy layer during the post-hot-dip cooling process. These stress-relieving cracks are typically extended during the post-galvanization drawing process. The presence of such fractures during cable processing further decreases the corrosion resistance of cables using such armor wires. Zinc can also flake off during cable manufacturing, leading to significant accumulation of zinc dust in the manufacturing area.
Commonly, sour well cables constructed completely of corrosion resistant alloys are used in sour well downhole conditions. While such alloys are well suited for forming armor wires used in cables for such wells, it is commonly known that the strength of such alloys is very limited.
Thus, a need exists for electric cables that are high strength with improved corrosion and abrasion protection, while avoiding cracking and accumulation of zinc dust in the manufacturing environment. An electrical cable that can overcome one or more of the problems detailed above while conducting larger amounts of power with significant data signal transmission capability, would be highly desirable, and the need is met at least in part by the following invention.